The present invention relates generally to a treatment for wastewater. More specifically, the present invention provides an enhanced system and more efficient process of treating industrial wastewater to remove ammonia and fluoride sequentially by a process of chemical destruction of ammonia, treatment with near stoichiometric amounts of calcium salts, and filtration.
Buffered oxide etch processes are used in the semiconductor industry to process silica wafers. The buffered oxide etch process requires the use of high concentrations of ammonia fluoride or ammonium bifluoride which end up in the semiconductor wastewater. Fluoride must be removed from wastewater streams prior to discharge into the city water systems, and local discharge limits can be lower than 5 ppm. Discharge limits for ammonia, which can be as low as 5 ppm, also often require its removal from wastewater. Additionally, ammonia interferes with the precipitation of calcium fluoride, necessitating the use of large excess amounts of calcium salts in the treatment process.
To meet discharge limits for ammonia and/or fluoride in wastewater, various treatment processes have been developed. Air stripping is one method of reducing ammonia concentrations in wastewater. Air stripping must be done at elevated alkaline pH and, to achieve very low concentrations of ammonia, in heated solution at pH greater than 10. This is very energy intensive, requires the addition of large amounts of caustic, and should be done without solids in the wastewater. Furthermore, rather than chemically destroying the ammonia, this process merely transfers it to the gas phase from which it must be scrubbed into an acidic absorption solution or discharged to the environment. Fluoride is not amenable to air stripping, and thus must be removed in a separate step, either before or after the air stripping.
Reverse osmosis (RO) is another available wastewater treatment technology that can be applied to treat both fluoride and ammonia. However, RO""s sensitive to membrane fouling by scale formation or biological growth on the filter membrane. Scale formation is avoided by either removing calcium, magnesium, and other scale forming materials prior to the filtration step or, more commonly, by acidifying the wastewater to deter precipitation of calcium and or magnesium carbonate or by adding dispersing agents to retard fouling at the RO membrane surface. Solvents, such as alcohols or ketones, and other sources of chemical or biological oxygen demand also must be removed to prevent biological growth on the membrane. The acidified RO concentrate preferentially contains both ammonia and fluoride, but as with air stripping, the ammonia is not chemically destroyed. Fluoride removal is normally done after the RO concentration step. However, the elevated ammonia concentrations in the concentrate hinders calcium fluoride precipitation necessitating use of large excesses of calcium salts.
Another treatment technique is ion exchange using cationic ion exchange resins. However, this process is not efficient for wastewater with high ammonia concentrations because the resins must be regenerated often at high chemical costs. As with the methods discussed above, ammonia is merely transferred from the wastewater to a different medium, in this case a concentrated acid waste stream. To avoid self-stripping of the ammonia, the pH and concentration of the incoming wastewater must be closely monitored and adjusted. Other cations such as calcium, magnesium, sodium, and the like will interfere with the removal of ammonia. Fluoride will interfere with effective removal of ammonia due to the formation of buffered ammonium fluoride compounds; and the ammonia will interfere with fluoride precipitation.
Biological treatment has been previously applied for destruction of ammonia in wastewater. However, such treatment is slow and relatively inefficient, requires the construction of treatment ponds with substantial capital cost and space requirements, is sensitive to temperature and ionic strength and the presence of other compounds in the wastewater, and requires organic matter and fertilizers to work well. Because only low fluoride concentrations are compatible with biological treatment, dilution of the wastewater can be required. Dilution prior to treatment wastes water and inhibits precipitation of fluoride because of limitations imposed by the solubility of calcium fluoride (about 16 ppm). Thus, the total mass of fluoride discharged from the treatment process increases with dilution since similar fluoride effluent concentrations are achieved through precipitation regardless of the starting fluoride concentration. This is the reason why it is more efficient to treat high concentrations of fluoride rather than low concentrations of fluoride.
Chemical destruction by hypochlorite has also been used to remove ammonia but with limited success. Precise pH control is necessary to avoid formation of chloramines. The presence of organic matter can lead to formation of halogenated organic compounds. Residual hypochlorite may generate toxic gases if mixed with unreacted ammonia solutions, for example, if the treated wastewater is mixed with other untreated rinse waters. Thus, an improved method for the removal of ammonia and fluoride is needed.
Accordingly, it is an object of the present invention to provide an improved system and method for removing ammonia and fluoride from wastewater. In particular, the inventor has found that a significant improvement in the prior art process for removal of fluoride from wastewater can be achieved by treating the wastewater with an oxidizer to chemically destroy ammonia before adding calcium salts to precipitate calcium fluoride. It is another object of the current invention to reduce the costs of removing fluoride from wastewater by reducing the chemical requirements and generating a low volume waste stream that can be more easily disposed of.
In general, the current invention present a process by which ammonia is chemically destroyed prior to treatment of fluoride. The wastewater is then treated with near stoichiometric amounts of calcium salts to precipitate calcium fluoride. Finally, suspended solids are removed from the wastewater. The final step can be a conventional settling tank or some sort of filtration system. A preferred solids removal process is a single pass, low pressure, high flow rate filtration step, such as the EnChem system available from MicroBar. Options include neutralization of excess oxidizer and byproducts, and removal of byproducts with activated carbon. The neutralization of excess oxidizer and byproducts can be done chemically prior to filtration. Often it is more convenient to do this step after filtration, when the solids-free water can be treated with reducing agents and/or contacted with activated carbon. Activated carbon can remove hypochlorites and chloramines by chemical absorption and reaction. Activated carbon also helps to remove any chlorinated organic materials which may have formed in the wastewater.
More specifically, the present invention provides a method for treating wastewater to reduce concentrations of fluoride and ammonia. Initially, in one embodiment, the oxidation-reduction potential of the wastewater is measured using an oxidation-reduction potential measuring device. Then an oxidizer is added in sufficient quantity to increase the oxidation-reduction potential of the wastewater above that of a chloramine solution. Next a soluble calcium salt is added to the wastewater at a concentration greater than stoichiometric. The soluble calcium salt lowers the solubility of fluoride in the wastewater and promotes precipitation of solid calcium fluoride. Excess oxidizer and oxidation by-products in the wastewater are then destroyed by adding an oxidizer and oxidation by-product neutralizing reagent to the wastewater. Alternatively, this can be done after the solids separation step. Finally the wastewater is passed through a solids separation system to separate the wastewater into a treated stream with a lower level of suspended solids and a concentrate with a higher level of suspended solids.
Among the advantages of the present invention are automation, efficient use of calcium salts, elimination of chloramine byproducts, and ease of use and filtration. The process can be applied in batch, semi-batch, or continuous-flow modes.